1. Field of the Invention
The present invention relates to a method and an apparatus for generating magnetic resonance images, and in particular to a method and an apparatus for generating magnetic resonance perfusion images.
2. Description of the Prior Art
Magnetic resonance technology has been increasingly used in recent years to generate angiographic images since, relative to other medical imaging modalities such as, for example, radioscopy with x-rays or computed tomography, it has among other things, the advantage that patient and medical personal are subjected to no radiation exposure.
Magnetic resonance (MR) technology is a known technology with which images of the inside of an examination subject can be generated. For this purpose, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and higher) in an MR apparatus so that the subject's nuclear spins become oriented along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to excite nuclear magnetic resonances, the excited nuclear magnetic resonances being measured (detected and MR images being reconstructed based thereon. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values by means of a multi-dimensional Fourier transformation.
Magnetic resonance can be used to produce images representing tissue perfusion, which is the flow of fluid in tissue. Perfusion studies allow an assessment to be made of the functioning of organs in vivo. For this purpose, in some techniques a contrast agent, which generates a signal that is detectable by magnetic resonance imaging, is injected into a subject, and the magnetic resonance data are acquired at a time when the contrast agent has optimally flowed into the region or anatomy of interest. Since the contrast agent is injected into the vascular system of the subject, the appearance of the contrast agent in the magnetic resonance image is representative of blood flow in the region or anatomy of interest. Magnetic resonance perfusion techniques are particularly useful in the context of magnetic resonance images of the head, in particular the brain, wherein cerebral blood flow (CBF) is identified. In another magnetic resonance perfusion technique using known arterial spin labeling (ASL) methods (which do not require injection of a contrast agent), images are often acquired with a perfusion-sensitive preparation (tag images) in alternation with non-perfusion-sensitive images (control images). The perfusion information in the tag images represents only a small change in the image contrast due to the inflowing tagged, i.e. magnetic resonance-labeled, spins into the region of interest, in the magnetic resonance images that are acquired. Typically, the perfusion signal is on the order of only a few percent of the total magnetic resonance image intensity.
Therefore, the extraction and quantification of relative perfusion images and quantitative perfusion images is prone to artifacts. Due to the low magnitude of the perfusion signal, multiple image acquisitions in the time frame of minutes are necessary. This results in a time series of images being acquired, with tag and control images alternating with each other. These tag and control images are combined with each other in pairs, by subtraction, so that multiple subtraction images are then available, which can be combined to form a resultant perfusion image. By combining multiple perfusion images, the low magnitude perfusion signal is made more readily visible in the combined image.
Due to the length of time that is necessary to acquire such an image series the most series source of image artifacts is patient movement, either gross (muscular) movement or natural movement such as respiration and cardiac motion. The control image is intended to be a static, snapshot image that can be ideally subtracted from the tag image that contains a small signal modulation originating from the perfusion. As noted above, this signal difference between the tag image and the control image is on the order of only a few percent. Artifacts can easily occur due to instabilities in the signal contribution to the control image, thereby causing the control image to deviate from a truly static image. In addition to non-static signal contributions originating within the examination subject, instabilities in the scanner that is used to acquire the magnetic resonance data may also cause the control image not to be truly static. An error as small as on the order of one percent in the control image can result in an artifact of approximately 100% false changes being attributed to the perfusion image.
As noted above, the typical processing that is employed to minimize this problem is to generate multiple perfusion images that are each a difference of a tag image and a control image. The multiple perfusion images are then averaged. A scaling or calibration factor can be applied to obtain a perfusion-weighted or a quantitative perfusion image. More advanced techniques use temporal interpolation methods of the time series of perfusion images, in order to recover the temporal resolution in the difference images. A typical procedure is to correct the original image series for motion before undertaking the averaging. This is accomplished by registering the image volumes of repeated measurements to a reference volume. This can significantly reduce the subtraction artifacts as long as the subsequent volumes can be fully registered. If motion artifacts occur within the volume, however, such volume-based registration fails, and leaves to significant subtraction artifacts, resulting in false perfusion images.